This invention relates generally to control systems for synchronous motors and more particularly to a control system for controlling the excitation to the rotating field winding of a synchronous motor without brushes.
Synchronous motors have a stator winding energized with a.c. current to produce a rotating magnetic field and a field winding energized with d.c. current to produce a unidirectional magnetic field which interacts with the rotating field to cause the rotor to rotate in synchronism with the frequency of the a.c. current. When starting a synchronous motor, the stator winding acts as a primary winding and the field winding as a secondary winding of a transformer. High voltages can be induced in the field winding which adversely effects the winding insulation and winding life. In order to eliminate this undesirable effect, the field winding is closed through a field discharge resistance during starting, and just before or after synchronization the field discharge resistance circuit is opened to avoid current drain from the excitation source.
Synchronous motor starting systems usually provide means for controlling the time that excitation is applied to the field winding. Most systems utilize means responding to the induced current in the field winding during the starting period. The induced current is sensed and when its frequency reaches a predetermined low value, the system operates to connect the excitation power source to the field winding.
A control system for a synchronous motor typically has a field discharge resistance circuit for discharging field current during the start-up period and a d.c. excitation circuit for energizing the motor field winding at synchronous speed as well as just prior to synchronization in order to develop the torque required to synchronize the motor. Rotor mounted rectifiers for supplying the d.c. excitation are well known in the art and semi-conductor switching means are used for controlling insertion of the field discharge resistance in circuit within the motor field winding whenever the induced field voltage rises to a predetermined value above the excitation voltage in order to prevent the voltage induced in the field winding during start-up from exceeding the peak reversed voltage rating and damaging the rectifiers and the winding insulation.
If the field is applied under the most favorable conditions of speed and rotor angle, the motor has the best chance of pulling into synchronism. If the direct current application is not precisely timed, the motor may slip back from synchronism and be tripped from the line, or the stator may momentarily draw greater power from the line than desired. The most favorable rotor angle occurs very near the point where the induced field current is zero and has just changed in polarity since at this moment the stator and rotor flux linkages are maximum and excitation current will build up rapidly if the d.c. is applied to the field.
U.S. Pat. No. 3,381,196 by Jerard M. LaRose, which issued on Apr. 30, 1968, discloses a control system for a brushless synchronous motor. The control system utilizes semi-conductor exciter switch means, such as an SCR, for applying excitation to the field winding which withholds excitation during start-up. The control system includes slip frequency and just as the slip voltage is reversing from a positive to a negative polarity. The control system uses semi-conductor switch means for inserting and removing the field discharge resistor and it applies a gating pulse to the exciter switch SCR which connects excitation to the field winding at a phase point of the slip voltage which assures reverse biasing of the field discharge resistor semi-conductor switch. Thus, the discharge resistance is removed when d.c. excitation current starts to flow into the field winding. When the SCR's are used to apply excitation to the field windind in this manner, the motor may fail to pull into synchronism if the SCRs are commutated off during the negative half cycle of the slip voltage. Further, the slip frequency sensing means of the control system may see a relatively high constant frequency voltage generated in the field winding under certain fault conditions, such as one phase of the stator winding being unbalanced, and such constant frequency signal prevents application of excitation to the field winding because the slip voltage frequency, as seen by the sensing means, never diminishes to the predetermined optimum fequency at which the gating signal is generated for firing the SCR.
U.S. Pat. No. 3,573,577 by Donald R. Boyd, which issued on Apr. 6, 1971, discloses a brushless synchronous motor control system which uses control semi-conductor exciter switch means which prevents the exciter means from being commutated off before the motor pulls into synchronism. The control system assures that the motor will pull into synchronism on reluctance starting even if a motor fault results in the generation of a constant frequency voltage in the field winding. The control system has a number of thyristors for connecting a d.c. source to the field winding and means for sensing when motor speed and rotor angle are most favorable for applying excitation. The system also has oscillator means controlled by the sensing means for applying a succession of gating pulses to the thyristors to prevent them from being commutated off before the motor pulls into synchronism. Semi-conductor switching means control insertion of the field discharge resistance in circuit with the field winding and an impedance in shunt with the switching means is selected so that the current through it and the field discharge resistance in series is greater when the switching means is open than the holding current of the thyristors thereby assuring that the thyristors are not commutated off before the motor synchronizes. The prior art exciters contain five SCR's which generate heat during operation. This heat must be dissipated using a heat sink which makes the structure more elaborate. It can be appreciated that it would be highly desirable to provide a brushless exciter which uses fewer than five SCR's and uses a simple heat sink. It is also highly desirable to have a simple exciter which can be controlled directly and one in which the amount of excitation can be controlled. It is also highly desirable to have an exciter which provides for field removal and reapplication during pull-out and resynchronization.
Accordingly, it is an object of the present invention to provide an apparatus for controlling the excitation of a synchronous machine without using brushes.
Another object of the present invention is to minimize the number of power components required for the exciter.
Still another object of the present invention is to provide a control which provides for field removal and reapplication during pull-out and resynchronization.